1. Field of the Invention
This invention relates to a solid electrolytic capacitor, such as tantalum capacitor or aluminum capacitor, which comprises a capacitor element enclosed in a resin package.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-open No. 2(1990)-105513 for example, a typical solid electrolytic capacitor comprises a capacitor element which includes a capacitor chip and an anode wire projecting from the chip. The capacitor also includes an anode lead in electrical connection with the anode wire, and a cathode lead in electrical connection to the chip. The capacitor element together with part of the respective leads is enclosed in a resin package, and the respective leads are bent outside the resin package for conveniently mounting to a surface of a circuit board (not shown).
The capacitor chip is conventionally prepared in the following manner. Specifically, an amount of tantalum powder is first compacted into a porous body together with the anode wire, and the compact is then sintered. Then, the sintered porous compact is oxidized to form a dielectric substance (e.g. tantalum pentoxide). Then, the oxidized porous compact is subjected to a treatment for forming a solid electrolytic substance (e.g. manganese dioxide). Finally, the thus treated compact is graphitized and metalized (by silver or nickel plating for example) for forming a cathode layer.
As is well known, a solid electrolytic capacitor is a polar component. Thus, if a reverse bias is applied to the capacitor element due to reverse polarity mounting of the capacitor relative to a circuit board or due to some trouble in the circuitry, the capacitor element may be dielectrically broken down when the reverse bias exceeds the reverse dielectric breakdown voltage of the capacitor element.
The reverse dieletric breakdown voltage of the capacitor element may be increased by increasing the layer thickness of the dielectric substance formed in the capacitor chip. However, such a solution results in a capacitance decrease of the capacitor because the capacitance of the capacitor is inversely proportional to the layer thickness of the dielectric substance. An attempt to decrease the layer thickness of the dielectric substance for increasing the capacitance of the capacitor will cause the capacitor element to dielectrically break down even at a relatively low reverse bias.